Electrochemical Migration Failure under Biased HAST due to Photoresist Residues

Abstract

Semiconductor package design density is increasing, thinner and smaller Cu trace is required. Finer Cu trace pitch is to be decrease insulation resistance among circuit patterns, which will induce to electrical short of circuit patterns by electrochemical migration when it expose to long time in high temperature and high humidity. Electrochemical migration behaviour has remained a major reliability concern. Electrochemical migration will also have a significant effect whenever charged ions are diffusing into a structure. If chloride ions are diffusing through dielectric layer, this will create a copper particle composition. In our study, the effect of photo-resist strip conditions on electrochemical migration was investigated. For this, we fabricated that specimen had photoresist residues on circuit patterns. Electrochemical migration test was conducted under the condition of DC 3.14 V, 130°C and 85% relative humidity. Life time prediction of specimens was used by the weibull analysis. And their failure mode was investigated by FIB (Focused Ion Beam) cross-section analysis. It was found that CuxO/Cu(OH)2 colloids and Copper dendrites were formed between short failed circuit nets. The mechanism of electrochemical migration was discussed depending on the existence of their residues after strip process of photo-resists. The Acceleration Factor (AF) which is the ratio of the life at use conditions to the accelerated life at test conditions for temperature humidity testing is given by the following Arrhenius-Peck equation. When assuming a product that has normal operating limits of 35°C and 95% RH and a typical activation energy of 0. 7eV,AF is calculated 302.3. This means that 1 test hour is equivalent to 302.3 hours in the field. Testing for 96hrs at the test condition would be equal to 3.3 years at normal use condition.